Implement `regexec2` using `stri_locate_first_regex`

[gagolews]

stri_locate_first_regex does not distinguish between a non-existing capture group and a no-match to a capture group in the case of many patterns. Also, it does not return capture group names.

Until https://github.com/gagolews/stringi/issues/424 and/or https://github.com/gagolews/stringi/issues/127 is resolved, the current version of regexec2 must be implemented based on stri_locate_all_regex, and this includes some performance penalty.